What’s up with the spiral arms of our galaxy?

The short answer:

This is awesomely weird. Imagine a bike wheel – each spoke is an “arm.” When the wheel rotates, the arms rotate, right? This is NOTHING like our galaxy. Imagine a beanie hat with a propeller on top. Hold the propeller still, and spin the hat underneath. That’s a little more like it. The hat is the stars, dust, gas, and everything our galaxy is made of. It spins around, passing THROUGH the arms (the propeller) of our galaxy. The arms aren’t made of anything. Whoa, what?

Okay, interested? Confused? Ready for more? Good, because that wasn’t a very accurate description, it was just enough to make you think.

Arms are Denser Areas

When I said “the arms aren’t made of anything” that’s because they’re just denser areas of the same stuff the whole galaxy is made out of, but as the galaxy spins, the arm is made up of different stars, different dust from one eon to the next. The stars and stuff glide right through the arms, though they may slow down while they’re there.

Try this at home:

Take a face-on mug shot of your favorite spiral galaxy (how’z about the Milky Way?)

Mark your 1000 favorite stars in one arm.

Wait 100 million years.

Take another photo.

Find those 1000 favorite stars, and where the arms are now, compared to before.

The arms may have moved a bit, but the stars will have moved a LOT. Some of them will be halfway around the galaxy again, some will only have moved a touch, and still others may have gone around a couple times. The arms will now be made up of different stars, but they’ll still be there.

Okay, obviously you can’t really do this at home for about 10 very good reasons, but you get the idea.

Stars Travel through the Arms (Traffic Jam)

Think about it another way. Spiral arms are like traffic jams of stars. You know how there’s always a traffic jam at the Renton S-Curves, or by the U District on I-5? If you take either of these routes, you’ll be in the traffic jam for a while, and then once you’re out of the bad area, you can get back up to speed. The traffic jam is still there though? (usually true … we’re talking about I-5 here), but you’re out, it’s someone else’s turn to bite their lip and grumble at traffic.

Same way with stars. The traffic jam is always there, but different people are in it at different times.

Density Wave Motion

Now that we’ve established that spiral arms are density waves (technically a “spiral density wave”), and it’s the stars that move through it, not the arm spinning around like a bike wheel, I’ve got a shocker for you. The arms move too, just at a different speed. That’s called the “pattern speed.”

Watch this video – see how the traffic jam moves around the circle, but the cars move differently?

Now this one – it’s a little easier to see both motions with this spinning galaxy.

Density Wave Formation

How do the spiral arms get started? Umm… that’s an open question, but I can tell you a couple of useful things.

First – the galaxy rotates differentially. Oops, jargon. The middle rotates at a different speed than the outer edge. Ta-da! Differential rotation. The Sun and Jupiter do this too. A bicycle wheel does not.

This differential rotation is due to Kepler’s laws. objects closer in revolve around the center a lot faster than objects further out.

Second, the likelihood that the galaxy started as one big perfectly even ball of gas is pretty low. There were pockets of lots of gas, and pockets of less stuff. These pockets will spread out and spin into denser areas.

Galaxy Rotation

So that’s a pretty easy explanation. There are still some problems we need to solve, but that should give you an idea.

Lastly, a way in which the arms keep themselves dense is due to the gravity of more material in the arm itself: stars speed up as they enter and arm, and then slow down as they leave it – the opposite of a traffic jam.

The Wind-Up Problem

But how come the arms don’t twist themselves up? Well, that’s one of the problems with the formation hypothesis I just showed you. This is the question that led to the idea of density wave arms in the first place.

It doesn’t twist itself up because the arms are not made of material, they’re just a denser area, there’s no reason for the arm to change.

2 Responses to “Density Waves: What’s Up with Galactic Spiral Arms?”

Thanks for the fascinating article Alice.
Beth has been forwarding this to me, and if your job is anything like hers was I know your swamped with work. But I woke up this morning with some questions on the subject and don’t know where else to go for answers.
As I understand it our solar system is in one of the arms of the Milky Way.

So what happens when we exit the arm?

Has anyone calculated how long we’ve been sitting in this stellar “traffic jam”?

Has anyone calculated our departure date?

Does this change the way we interpret the evidence for an expanding universe, since our movement can no longer be considered a constant?

I hope that PSC is recognizing the outstanding work you are doing on this site.

I don’t know. We’ll be in a less dense area – there will be less star formation going on around us – so over time perhaps we’ll see fewer young stars. (“We” is figurative here … we won’t actually be here, but the Earth will).